In recent years, multilayer composite films having two, three or four layers have been developed for use as packaging materials for foods and so forth. The composite films are produced by laminating one or more plastic films such as polyethylene, polypropylene, nylon, polyester and polyvinyl chloride films, and a metal foil, such as an aluminum foil. Such materials are in wide use.
For the adhesion of such plastic films or metal foils, use of an adhesive agent prepared by incorporating an oxy acid of phosphorus or a derivative thereof, epoxy resin and a silane coupling agent into a composition containing an organic polyisocyanate and an organic polyol has already been proposed [JPB S61(1986)-4864).
And, use of an adhesive agent prepared by incorporating a polybasic acid anhydride having at least two acid anhydride groups in the molecule into a mixture of an organic polyisocyanate and an organic polymer polyol has also been proposed [JPA S61(1986)-47775].
Further, use of an adhesive agent prepared by incorporating an oxy acid of phosphorus or a derivative thereof, a carboxylic acid compound or its anhydride and epoxy resin into a mixture of an organic polyisocyanate and an organic polyol has also been proposed [JPA H2(1990)-84482).
However, packaging materials produced by using such adhesive agents as mentioned above are not fully satisfactory in bond strength and are disadvantageously insufficient in heat resistance, or chemical resistance such as acid resistance and alkali resistance. For instance, when a package containing a free fatty acid-containing food showing high acidity is retort-sterilized with hot water of a high temperature of at least 135.degree. C. and subjected to a long-term preservation test, it was found that the metal foil-plastic film bond strength decreased and, at the same time, the strength of the packaging material itself decreased. In extreme cases, complete delamination takes place and causes the metal foil to have pinholes, which in turn decreases the air barrier property intrinsic to the metal, thus the intended long-term preservation of foods cannot be achieved.
Furthermore, when vinegar or a highly acidic food with a high free fatty acid content is packaged, the bond strength decreases during the long-term storage even in the absence of a high-temperature sterilization procedure such as boiling or retort, to cause delamination of the plastic film from the metal foil.